One solution to the hot corrosion and oxidation problem which is widely applied in gas turbine engines, is to allow aluminum into the surface of a superalloy component, a process known aluminizing. Aluminum forms stable intermetallic compounds with both nickel and cobalt. The oxide layer which forms on these compounds at high temperature is no longer a metal oxide of nickel or cobalt, but rather a tough, tightly adherent, protective layer of alumina, Al.sub.2 O.sub.3. See FIG. 3.
A variety of commercial coatings are based upon this protection scheme. Sometimes aluminum is deposited from a vapor phase in a process that has come to be known as pack aluminizing. In pack aluminizing, aluminum powder is reacted with halide activators to form gaseous compounds which condense on the metal surface and react producing aluminum metal. The aluminum atoms diffuse into the substrate, reacting to produce intermetallic aluminides. This process has been described in detail in a number of patents, including U.S. Pat. No 5,256,230 (Wochtell et al). This patent is incorporated herein by reference.
State-of-the-art MCrAlY overlay coatings also rely upon alumina films for their hot corrosion resistance. Owing to the presence of chromium and yttrium in the film, aluminum contents in these coatings do not need to be as high as in pack aluminides; however, protection is still derived from a tightly adherent scale of alumina.
Slurry aluminizing is another alternative method of providing a protective, alumina forming intermetallic aluminide coating on a superalloy. In the slurry process, an aluminum-filled slurry coating is first deposited on the hardware. When the coated part is heated in a protective atmosphere, aluminum in the film melts and reacts with the substrate to form the desired intermetallic phases.
The demonstrable resistance of aluminide coatings to hot corrosion and oxidation is due to the thermodynamic stability of the alumina scale that forms on them. However, they do have some susceptibility to "low temperature" hot corrosion attack at about 700-800.degree. C. by alkali metal oxides (e.g. Na.sub.2 O) and acidic oxides of refractory metals (e.g. MoO.sub.3 and W.sub.2 O.sub.3).
Silicon dioxide (SiO.sub.2) is another very stable oxide. Like aluminum, silicon forms stable intermetallic compounds (silicides) with nickel and cobalt as well as chromium and other elements typically found in refractory alloys, such as molybdenum, tungsten and titanium. This reduces the segregation of these elements into the outer surface protective oxide layer, thus improving its protectiveness. Furthermore, unlike aluminum, silicon is unable to form sulphides and is resistant to sulphur diffusion. Consequently, silicide coatings, produced by pack or slurry processes, have been used on refractory alloys to improve resistance to hot corrosion and oxidation. Silicides have proven particularly useful in resisting sulphurous attack at "low" temperatures (700-800.degree. C.). The benefits of silicon-based coatings have been described by many, including F. Fitzer and J. Schlicting in their paper "Coatings Containing Chromium, Aluminum and Silicon for High Temperature Alloys", given at a meeting of the National Association of Corrosion Engineers held Mar. 2-6, 1981 in San Diego, Calif., and published by them as pages 604-614 of "High Temperature Corrosion", (Ed. Robert A. Rapp). This paper is incorporated herein by reference.
For the avoidance of doubt, silicon is classed as a metallic element for the purposes of this specification.
The benefits of aluminizing and siliconizing are combined in processes which simultaneously deposit both aluminum and silicon on a metal surface, usually that of a superalloy. One such process, described in U.S. Pat. No. 4,310,574 (Deadmore et al), which is incorporated herein by reference deposits a silicon-filled organic slurry on a surface, then aluminizes the surface by a conventional pack aluminizing. Aluminum carries silicon from the slurry with it as it diffuses into the superalloy from the pack mixture. Deadmore et al ('574) demonstrates that the resultant silicon-enriched aluminide has better resistance to oxidation at 1093.degree. C. than did aluminides without silicon.
Another means to produce so-called "silicon-modified" or "silicon-enriched" aliuminides is to apply a slurry coating containing powdered aluminum and silicon metal to an alloy substrate containing aluminide and silicide forming elements and then heat it above 760.degree. C. (1500.degree. F.). As the aluminum and silicon in the slurry melt, they react with the substrate and diffuse preferentially. The aluminum alloys with nickel or cobalt in the substrate while silicon alloys with chromium or other silicide formers. The end result is a composite aluminide-silicide coating. This process is often termed a silicon modified slurry aluminide process and is commercially utilized under the trade name, "SermaLoy J", (a proprietary tradename of Sermatech International, Limerick, Pa., U.S.A.).
Generally speaking, these prior art techniques and coating compositions aim at increasing the silicon in the layer of the coating exposed to, the harsh conditions described.
Alloy substrates suited to this form of coating include nickel-based superalloys, cobalt-based superalloys and austenitic stainless steels. It is found that elements corresponding to the constituent elements of the alloy substrate are present throughout the extent of the coating but are combined differentially with the aluminum and silicon constituents of the coating such that the silicon rich phases are differently distributed through the thickness of the coating relative to the aluminum rich phases.
As supplied for use, the SermaLoy J slurry coating composition comprises silicon and aluminum elemental metallic powders in an acidic water solution of inorganic salts as a binder. About 15% by weight of the total metallic powder content of the slurry is silicon powder. However, the overall composition of the slurry in approximate weight percentages is
Al powder--35% PA1 Si powder--6% PA1 Water--47% PA1 Binder salts (dissolved in the water)--12%
A preferred mode of preparation of the composition is to premix the metallic powder constituents and make the binder solution separately, then mix the powder into the solution. Other ways of preparing the composition can readily be devised.
This binder is selected to cure to a solid matrix which holds the metal pigments in contact with the metal surface during heating to the diffusion temperature. It also is selected to be fugitive during diffusion to yield residues that are only loosely adherent to the surface after diffusion has been completed.
In tests, the silicon-modified aluminide coating resulting from application of the slurry to superalloy articles and subsequent diffusion heat treatment proved uniquely resistant to sulphidation attack over a wide range of operation temperatures. Details of some testing has been published by American Society of Mechanical Engineers (ASME) in a paper by F. N. Davis and C. E. Grinell entitled "Engine Experience of Turbine Materials and Coatings" (1982) which is incorporated herein by reference. This coating is now specified on many industrial and marine turbines.
The paper reports that SermaLoy J shows good resistance to low and high temperature sulphidation. Because of the very satisfactory properties of the SermaLoy J coating, it is used as one of the standards for comparison in the tests discussed herein.
The theoretical basis for the above improvement is believed to be as follows.
Diffusion heat treatment of an aluminum-silicon slurry coated superalloy or austenitic stainless steel substrate in an inert atmosphere or vacuum causes certain elements from the substrate and the slurry, which have a particular affinity for each other, to diffuse towards, and combine with each other.
At the diffusion temperature, aluminum from the coating and nickel and/or cobalt from the superalloy or stainless steel substrate move rapidly towards each other and combine to form nickel aluminides. Similarly, silicon in the coating has an affinity with the substrate metal chromium, and with molybdenum, tantalum and titanium, if present, and therefore, combines with one or more of these to form their silicides.
However, silicon moves through the coating towards the substrate appreciably more slowly than the aluminum and therefore the outer parts of the coating become relatively enriched with silicon. Because chromium is present in superalloys and austenitic stainless steels in much larger amounts than the other elements for which silicon has an affinity, this silicon mostly combines with chromium during the diffusion treatment to produce an outer coating layer which is richer in chrome silicide than the rest of the coating.
It is convenient for further reference in the description of the invention, to identify several zones in a typical SermaLoy J coating, diffusion heat treated at 870.degree. to 885.degree. C. Inspection of the coating shows a silicon-rich surface zone where chromium silicide is particularly concentrated. This zone transitions to a "layering zone" extending deeper into the coating, comprising alternate layers of silicide and aluminide phases. Beneath the layering zone is an "aluminide zone", where aluminide phases predominate, but also containing silicide precipitates. At the interface with the substrate material, there is a diffusion zone, where the coating and substrate materials have diffused into each other.
The coatings of the invention, as is discussed herein, show a different composition of these layers and/or different thicknesses of the layers.